"Contour fonts" is a term that refers to the use of outlines or contours to describe the shapes of characters used in electronic printing. In a contour font, each character shape is represented by one or more closed curves or paths that traces the boundary of the character. The contour is specified by a series of mathematical equations, which may be in any of several forms, the most common being circular arcs, straight lines, and polynomial expressions. The shape of the contour font is that of the ideal design of the character and, generally, does not depend on parameters associated with any printer. Contour fonts are ideal for use as master representations of typefaces.
Bitmap fonts or raster fonts are composed of the actual characters images that will be printed on a page, and are made by scaling contours to the appropriate size, quantizing or sampling them at the resolution of the printer, and filling the interiors of the characters with black bits or pixels. Achieving high quality in this process is difficult, except at very high resolutions, and requires knowledge of both the marking technology and typographic design considerations. Often, a bitmap font is delivered to a printer. There is a separate bitmap font for each size of a font, and sometimes separate fonts for landscape and portrait orientations.
The advantage of a contour font is that it can be scaled to any size and rotated to any angle by simple mathematics. Therefore, a single font suffices to represent all possible printing sizes and orientation, reducing font storage requirements, reducing the cost of font handling.
The difficulty in this approach is in achieving high quality character images during the sampling process which generates the raster characters from the contour masters. If the contour character is simply sampled, there will be random .+-.1 pixel variations in stroke thickness. If the printing process tends to erode black areas (common in write-white laser xerography) characters will be consistently too thin. If the printing process tends to fatten black areas (common in write black laser xerography), characters will be too thick.
At the high resolution employed in phototypesetters, usually greater than 1,000 spi, no special techniques are required for scaling and sampling the contour font to generate a raster font of any size. This is because although simple sampling necessarily has random one-bit errors, such errors are small compared to the size of the character, making errors insignificant. At 300, 400, and 600 spi though, character strokes are only three or four bits thick and each bit is important. The simplistic methods used by typesetter manufacturers are not sufficient.
U.S. Pat. No. 4,675,830 to Hawkins, uses defined points in a contour font that must be grid aligned to pixel positions, but the stem widths or edges are not aligned.
Of particular importance in generating fonts of optimal appearance are maintenance of uniform and correct stroke thickness among characters of a font and on different printing engines, uniform alignment of characters on a baseline, and uniform spacing of characters.